When the spread is wider on an ECN than on the exchange (say in the qqq's), it isn't anything to complain about. It is simply that there are no either no sellers at the offer, or buyers at the bid. Remember, you may complain about specialists on the floor, but on the ECNs, we make the market. If we are not there, there is no market. The enemy is us

A safeguard does exist however. If you put in a market order on an ECN, and there is no price equal to or better than the exchange price, you just don't get a fill...not until the prices move to the market, or new bids/offers come in. But you will never be filled at a worse price.
I have often watched the prices of the qqq's on my level 2 instinet terminal while watching the prices on the amex. Occasionally you can arb the difference if you are fast enough. I have to believe however (though I don't know for certain), that the specialist has to be looking at the same thing, so it is a rare occurance to catch them flatfooted. Personally, I can't type or click fast enough to route my orders to take advantage of these situations. But I do work alongside some guys that do it. In better markets, I would say it isn't worth the effort to scalp a penny or two (or three or whatever....). But these days, every little bit helps. Too bad I am a bit older than the Nintendo generation. I think that quick fingered video game talent comes into play on this kind of thing.
When the specialist's spread does widen, making it easier to "middle" on an ECN, it is most likely due to a faster market. So even if you see an opportunity, while it exists for someone, it is unlikely to be you. Remember that the ECN's (the one I get to see stand alone is INCA) really are limited in size. You may see a bid or offer you want inside the specialist's spread, but what good is it if you go market on say 1000 shares and the price you want is only good for 100? Or worse, an odd lot (not on INCA, but certainly possible on say ISLD) You then get filled on the other 900 at the specialist's price anyway (in most cases). By the time that happens, you may not want the trade. And even the fastest can't type in a limit order price fast enough to avoid that. And if they do, again, the trade is probably going the wrong way by that time.
My thought is basically if you have conviction about the trade, don't sweat the penny (pennies). Why miss a trade with an unfilled limit order when a market order would have given you the certain fill?
Bear in mind, this is coming from a guy who trains traders to use limit orders virtually always to open positions. But the reason for that is I don't want the new traders to chase, so I have them ALWAYS open long trades by entering "bid" in the price box, eliminating them from buying into up movement, and "offer" when opening shorts. But that is just to establish good habits for trainees (which we all are for years really). Having said that, after you have disciplined yourself, sometimes you just want the trade beause you really believe in it. Timing then doesn't need to be perfect. That is when you should use market orders anyway (to open....closing trades, well sometimes you just NEED to get out).
And I agree with Don....the specialists don't want to ruin the good thing they have going. Hopefully the day will come (sooner the better) when they make themselves obsolete. Until then, they are just a part of our cost of doing business.